1. Field of the Invention
The present invention relates to an electric power tool such as an impact driver.
2. Related Art
An impact driver includes a housing, a hammer case received in the housing, and an inner case received in the hammer case. The housing accommodates not only the hammer case and the inner case, but also a motor having a drive shaft, a switch and the other structural components. The inner case accommodates a planetary gear train having an input side and an outputs shaft, which is connected to the drive shaft of the motor to transmit power of the motor to the output shaft at reduced speed. The hammer case accommodates a hammer unit having an input side and an output shaft. The input side of the hammer unit is connected to the output shaft of the planetary gear train to convert rotational motion of the output shaft of the planetary gear train into an intermittent striking power outputted from the output shaft of the hammer unit. Japanese Patent Provisional Publication No. 2003-145439 describes such a structure of the impact driver.
In the conventional impact driver, the inner case is inserted into the hammer case, and then the hammer case is secured to the housing by means of screws. The housing and the hammer case are provided on their peripheries with bosses. The corresponding bosses are fastened by screws so as to fix the hammer case to the housing.
In addition, the inner case is fixed to the housing by means of screws, and the hammer case is fixed to the inner case by means of screws (see the above-mentioned Japanese Patent Provisional Publication No. 2003-145439).
However, the former conventional impact driver has the bosses projecting outward so as to hinder an operator from carrying out a working operation at a corner or narrow portion to be worked or to damage a material made for example of wood into which screws are to be inserted.
In addition, the latter conventional impact driver involves the following problems:    (1) The hammer case is connected indirectly to the housing through the inner case, thus leading to a possible deterioration in securing force by which the hammer case is supported on the housing.    (2) The hammer case is fastened to the housing by turning the screws into threaded bores formed in the hammer case, with the result that an intermittent striking power generated by the hammer unit and its reaction force are transmitted through the housing and the hammer case to the screws in the form of rotational force to loosen them.    (3) It is not easy to determine a relative position among the housing, the inner case and the hammer case, thus leading to a complicated assembling or disassembling operation.    (4) The inner case and the hammer case are connected in this order to the housing. When an inspection or repair for example of switching elements accommodated in the housing is carried out, the housing cannot be opened until the hammer case is removed from the inner case and then, the inner case is removed from the housing. Accordingly, the inspection or repair operation becomes complicated.